Projection lens systems for cathode ray tube (CRT) projection televisions have undergone continuing development during the past fifteen years or so. Examples of such systems can be found in Betensky, U.S. Pat. Nos. 4,300,817, 4,348,081, 4,526,442, 4,697,892, and 4,801,196; Moskovich, U.S. Pat. Nos. 4,682,862, 4,755,028, and 4,776,681; and Toide, U.S. Pat. No. 5,148,320.
Color images for projection televisions are normally obtained by combining images from three color CRTs, i.e., a red CRT, a green CRT, and a blue CRT. The phosphors used in commercially available CRTs do not emit light at a single wavelength. In particular, green phosphors have significant sidebands in blue and red. Similar polychromaticity exists for red and blue phosphors, but to a lesser extent.
For many consumer applications, lens systems uncorrected for color can be used, notwithstanding the color spread of the CRTs. For more demanding applications, however, such as high definition television, data displays, or systems which operate at a high magnification, color correction is needed to avoid visible color fringing and/or a loss of image contrast. Examples of projection lens systems which provide at least some color correction include Betensky, U.S. Pat. No. 4,815,831; Kaneko et al., U.S. Pat. No. 5,404,246; Kreitzer, U.S. Pat. Nos. 4,900,139, 5,309,283, and 5,455,713; Moskovich, U.S. Pat. No. 4,963,007; Toide, U.S. Pat. No. 5,130,850; and Yoshioka, U.S. Pat. No. 5,237,456.
A particularly demanding application for projection televisions is in the area of simulators, e.g., flight simulators, where the goal is to produce an image which mimics real life as closely as possible. Performance requirements for such systems can include an f/# of less than 1.0, a total field in the direction of the image (screen) of as much as 90.degree., full color correction over the 465 to 610 nanometer range, and a modulation transfer function (MTF) greater than 50% at 10 cycles/millimeter through 0.85 of the total field.
In addition, to enhance the brightness of the image, it is desirable to achieve these performance characteristics for relatively large CRTs, e.g., CRTs which have a diagonal on the order of 160 millimeters. Along these same lines, it is desirable to minimize vignetting of the light passing through the projection lens system so as to increase the amount of light which reaches the viewing screen, e.g., it is desirable to keep vignetting losses below 30% at full field. As known in the art, vignetting can be used in the design of an optical system to remove off-axis rays which if allowed to reach the screen would degrade the quality of the image. Minimizing vignetting thus puts even higher demands on the basic performance of the lens system since with vignetting minimized, more rays reach the screen and thus must be corrected.
In addition to these considerations, the projection lens system should be focusable over a range of conjugates, e.g., a range of approximately .+-.7.5% from a center value of about 4 meters. Such focusability provides flexibility in the types of applications in which the system can be used and in the set-up procedure for any particular installation. The lens system, of course, must continue to meet the above performance characteristics as it is focused over such a conjugate range.
Although of high quality, the existing projection lens systems are not able to meet all of the above criteria. There thus exists a need in the art for an improved projection lens system which is capable of satisfying these criteria. It is an object of the present invention to provide such a lens system.